It is well known that the laser diode is a typical example of a current injection type light-emitting element. When a forward drive current flows in through the laser diode, electrons and holes recombine to generate photons in the region of the active layer, and the spontaneously emitted light is fed back and forth in the active layer to generate laser oscillation (stimulated emission). As shown in FIG. 7, the drive current-optical output characteristic curve of the laser diode contains an inflection point called the oscillation threshold or threshold current ITH. When a drive current higher than said threshold current ITH is supplied (injected), laser oscillation (light emission) occurs, so that the optical output can be controlled and adjusted easily and at high speed.
One application of the laser diode is its use in printing heads. A laser system used in a printing head has a constitution in which the beam spot of the laser beam generated at a prescribed dot period from the laser diode is scanned over the photosensitive drum in the line direction (principal scanning direction). In this case, the laser diode generates a high-speed pulsed optical output in response to a high-speed pulsed signal.
In order for the laser diode to emit light, it is necessary to increase the drive current from zero; when the drive current exceeds theoretical current ITH, laser oscillations (light emission) are initiated. However, there exists a delay time for the drive current to reach threshold current ITH. In applications in which the laser diode is driven with high-speed pulses, a DC bias current set near the threshold current ITH is injected into the laser diode. During the pulse period, a constant switching current that rides on the bias current is supplied (injected), and the optical output of the laser diode increases rapidly, following the switching current. In this case, corresponding to the drive current versus optical output characteristics, an optical output is obtained corresponding to the drive current or the sum of the bias current and the switching current.
However, since the bias current is fixed, if the drive current versus optical output characteristics of the laser diode vary due to changes in ambient temperature and other changes over time, an offset between the bias current and threshold current ITH can occur, and as the offset becomes larger, problems arise. That is, when threshold current value ITH changes to larger value than the bias current, a delay takes place for the start of laser oscillation after injection of the switching current, or the delay becomes longer. On the other hand, when threshold current ITH changes to a smaller value than the bias current, even when the switching current is not injected, light will be generated all the time, although it is weak.
In order to solve the problems of the aforementioned fixed bias method, the following scheme has been proposed: the drive current versus optical output characteristics of the laser diode is represented using a linear approximation to compute the theoretical value of threshold current ITH, and the current value obtained by extrapolating back from a prescribed value is used as the bias current in the variable bias method. In this variable bias setting method, a plurality of operating points (usually 2) on the drive current versus optical output characteristics are measured, and the point of intersection of the straight line defined by said two operating points to the abscissa (current axis) with optical output of zero is determined. The current at the point of intersection is used as the theoretical threshold current value ITH. Even when the drive current versus optical output characteristics of the laser diode varies, it is still possible to automatically reset or update the bias current tracking said variation.
However, the drive current versus optical output characteristics of the laser diode are not strictly a straight line even in the oscillation region where the current exceeds threshold current ITH. In said conventional variable bias setting method, because the aforementioned nonlinear characteristic curve is approximated by a straight-line fit, the precision of determining threshold current ITH determined on the basis of this theory is reduced, and thus the precision of determining the bias current is also reduced.
In addition, according to the aforementioned variable bias setting method, in order to determine plural points on the drive current versus optical output characteristic curve, it is necessary to perform at least two cycles of operation for the operating mode in setting the bias current by measuring the current value corresponding to the optical output at a prescribed value as the laser diode is temporarily turned on. This leads to a significant restriction on the applications. That is, in order to keep a constant optical output at the normal operating point with respect to variation in the drive current versus optical output characteristics of the laser diode, it is necessary to reset not only the bias current but also the switching current. Consequently, after the operation for setting the bias current, it is necessary to perform an operation for setting the switching current that establishes the switching current for obtaining the nominal operating point as the laser diode is temporarily driven under a new bias current. Thus, it is necessary to perform a total of three or more cycles of operation in the operating mode (temporary driving) for resetting the bias current and the switching current. However, for printing heads with laser systems, the times that said resetting mode can be used are limited to the scanning period (such as the flyback line period), and if the time needed for the resetting mode becomes longer, it becomes impossible to use either method, or the number of executions must be reduced.
In addition, for the light-emitting element driver on the basis of the aforementioned variable bias setting method, there should be not only a complicated analog operation circuit for determining the theoretical threshold current and bias current using the straight line approximation method, but also special current generating circuit which is different from the bias current supply circuit used in the bias current setting mode, for generating the bias current determined in another operation. As a result, the circuit scale is very large, which is undesirable.
A general object of the present invention is to solve the aforementioned problems of the prior art by providing a light-emitting element driver characterized by the fact that the bias current is automatically set very near the threshold current of the current injection type light-emitting element, and it can guarantee a stable optical output and high-speed initiation of the light-emitting operation.